1. Field of Invention
The present invention relates to a gas separating asymmetric membrane comprising an aromatic polyimide. Particularly, the invention relates to a gas separating asymmetric membrane (including a gas separating asymmetric hollow fiber membrane) exhibiting an excellent gas transmission property (e.g., gas transmission rate) and an improved mechanical property. The invention also relates to a process for the preparation of such excellent gas separating asymmetric membrane.
2. Description of Prior Art
It has been previously known the process to prepare an asymmetric gas separating membrane (including an asymmetric hollow fiber membrane) having a homogeneous surface layer on one side and a porous layer on another side by a wet membrane-forming method. In the method, a coagulation liquid and a dope solution of an aromatic polyimide made from the aromatic tetracarboxylic acid component comprising mainly biphenyltetracarboxylic dianhydride and an aromatic diamine component comprising mainly an aromatic diamine having --SO.sub.2-- group in its molecular structure are used. For example, the process is disclosed in U.S. Pat. No. 4,690,873.
A gas separating asymmetric membrane of the biphenyltetracarboxylic acid-type aromatic polyimide not only has good heat and chemical resistance but also shows excellent gas selectivity, particularly, for collecting carbon dioxide gas from a gaseous mixture. However, the membrane does not always exhibit satisfactory gas transmission rates when it is used for separating various gaseous mixtures in practical uses. Therefore, it is desired to develop a gas separating asymmetric membrane exhibiting improved gas transmission rate.
In U.S. Pat. No. 3,822,309, No. 3,899,309, No. 4,705,540 and No. 4,717,394, a polyimide gas separating membrane of aromatic polyimide in the form of a homogeneous body which is prepared from an aromatic diamine having one or two benzene rings and an aromatic tetracarboxylic acid component such as pyromellitic dianhydride, benzophenonetetracarboxylic dianhydride, biphenyltetracarboxylic dianhydride or 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanic dianhydride (hereinafter referred to as "6-FDA") is disclosed.
As for 6-FDA, it is apt to give a polyimide of an unfavorably low molecular weight when it is caused to react with an aromatic diamine having one benzene ring such as 1,3-diamino-4,6-dimethylbenzene. Otherwise, 6-FDA cannot give a polyimide gas separating membrane having both high gas permeation ratio and high gas selectivity, when it is caused to react with an 4,4'-diaminodiphenylether or 4,4'-diaminodiphenylmethane.
In "Polymer Preprints", Peggy Cebe et al, Vol. 29, No. 1, 130 (1988), it is described that polyimide molding (e.g., film) using 6-FDA as an acid component is inferior to others in mechanical property from measurement of mechanical characteristics of various aromatic polyimide films. In practice uses, a gas separating asymmetric hollow fiber membrane consisting of 6-FDA-type polyimide is apt to be destroyed or ruptured, when it receives a relatively strong pressure in a procedure for its preparation process or its converging procedure for module of fiber. Therefore, it is desired to develop a gas separating membrane having both an excellent gas transmission property such as high gas transmission rate and selective permeability (selectivity) and an improved mechanical property.
Hence, U.S. Pat. No. 5,042,992 discloses a polyimide gas separating asymmetric membrane of aromatic polyimide which is prepared from 6-FDA and 3,7-diamino-2,8-dimethyldiphenylenesulfone. According to the study by the present inventors, however, the membrane does not also sufficiently satisfy properties of both the gas transmission property and the improved mechanical property, particularly does not exhibit excellent mechanical property.
Further, the specification of the application Ser. No. 835,605, filed Feb. 13, 1992 now abandoned, describes a polyimide gas separating asymmetric membrane of aromatic polyimide which is prepared from 6-FDA and a sulfur atom-containing diamine such as diaminodibenzothiophene, diaminodiphenylenesulfone or diaminothioxanthene-5,5-dioxide and an aromatic diamine having at least two benzene rings. The above application is a continuation-in-part of Ser. No. 669,523, filed Mar. 14 1991, now abandoned.